This invention relates to the continuous formation, by extrusion, of a product in which constituent elements, normally randomly oriented, are capable of alignment or other ordered orientation to enhance the strength, formability or other useful quality of the extruded product. The invention applies particularly to the formation of extruded products from composite materials comprising a plastics base, for instance polypropylene or nylon, and a fibrous filler such as glass or carbon fibre or a particulate organic or inorganic filler, for example ceramic particles in the form of platelets. Examples of such compositions are described in our patent specification GB-B-2085461. The invention also applies, however, to the continuous formation of products from homogeneous materials, the molecular or other structure of which is capable of undergoing useful physical orientation. The inventions thus applies, for example, to the formation of products from thermoplastic semicrystalline polymer materials which can be effectively oriented, such as homo- or copolyolefins. It also applies to the removal of unwanted orientation effects which may be produced in conventional extrusions, for instance where a melt has to be separated and then recombined within the die prior to expulsion.
Methods and apparatus for achieving useful alignment of fibres or other constituents within moulded products have already been proposed, for example in our patent specification GB-A-2170142, but the process of continuous formation by extrusion poses problems which do not have to be addressed in the formation of a one-off product within a closed mould. Amongst these problems, of course, is the continuous movement of the product and its change of state as it moves.